This invention relates to a magnetic recording medium, and in particular relates to an improvement in the running durability of magnetic recording media using ferromagnetic metal powder.
In recent years, there has been increased demand for magnetic recording media suitable for high density recording. In this regard, magnetic recording media comprising ferromagnetic metal powder and a binder (hereafter, metal powder type magnetic recording media) are superior in magnetic properties such as coercive force (Hc) and saturation magnetization (Bm), and may be expected to serve as magnetic recording media suitable for high density recording. Indeed, they are already being presented for practical use in 8 mm video tapes and video floppy disks magnetic recording media.
However, the metal powder magnetic recording media require smooth magnetic layer surfaces in order to handle high density recording, so there is a tendency toward an increase in the coefficient of friction for the contact between the apparatus system and the magnetic layer of the running magnetic recording medium, stick slips occur, output becomes unstable because of head vibration, head sticking occurs, and running durability of the tape deteriorates.
Various attempts have been made to improve upon such problems and to raise the running durability of the metal powder type magnetic recording media.
For example, there are methods of adding lubricants in the magnetic layer, such as the method of adding polyethylene glycol ester (JP-A-57-3225), the method of adding ketone compounds (JP-A-57-15802), the method of adding a fatty acid glycerol ester and a fatty acid sorbitan ester (JP-A-57-158027), the method of adding a fatty acid and a fatty acid ester (JP-A-58-130435, JP-A60-5420 and U.S. Pat. No. 4,465,737), and the method of adding a fatty acid alkyl ester and fatty acid alkoxy ester (JP-A-61-229234, JP-A-61-230624 and U.S. Pat. No. 4,652,500) (The term "JP-A" used herein means an unexamined published Japanese patent application). However, with these method, it is difficult to regulate or select the amount of the lubricant which should be added, because when the lubricant is added in an amount where the lubrication effect appears, the film quality of the magnetic layer becomes weak, its durability deteriorates, and sticking occurs at the head.
Also, as a different method, there is the method of adding abrasives in the magnetic layer. For example, there may be mentioned the method of adding .alpha.-ferric oxide (JP-A-58-159227), the method of addung Al.sub.2 O .sub.3 (JP-A-58-171721), and the method of adding particles under 0.2 .mu.m in size and over 6 in Mohs' hardness (JP-A-61-289528 and U.S. Pat. No. 4,687,703). However, when the abrasive is added in an amount sufficient to raise the running durability, other problems appear. Such problems include, for example, the fact that the magnetic head wears out, the surface qualities of the magnetic layer worsen, and electromagnetic conversion properties deteriorate.
Still another method that has been proposed is exemplified by the method of combined use of an abrasive and a fatty acid ester (JP-A-58-189826), and the method of combined use of carbon black and a silicone oil (JP-A-62-195730).
However, with every one of these method, there has been an accompanying decline in other properties, and they have not been sufficient to the point where they are capable of improving upon the running durability of metal powder type magnetic recording media to any degree. Particularly in cases when the attachment and detachment of the head to the magnetic layer surface is repeated many times, as with floppy disks, the coefficient of friction of the magnetic layer increases during travelling so that the output is made unstable by head vibration due to stick-slip of the medium, sticking on the head leads to poor running, and it has been very difficult to solve these problems by the conventional method.